Although applicable to any optical sensor assemblages, the present invention and the problem on which it is based will be explained with reference to a thermopile sensor assemblage.
Thermopile sensors belong to the group of thermal radiation sensors, i.e. an incident radiation results in a temperature difference between a receiver surface and a difference region (heat sink). This temperature difference is converted into an electrical voltage with thermoelements, on the basis of the Seebeck effect. The output voltage depends not on the geometric dimensions of the thermoelements, but only on the material combination selected. The output voltage of the thermopiles thus implemented is increased by serially connecting multiple identically irradiated thermoelements.
Using micromechanical technologies, it is possible to manufacture the thermopiles as silicon chips, similarly to conventional semiconductor components and using typical semiconductor processes. The application of multi-level technologies additionally permits a further reduction in the geometric dimensions of the thermopiles. To achieve maximum sensitivity, the thermopiles are provided on a thermally insulating membrane that is as free-standing as possible. An etched silicon frame serves as carrier for the membrane and simultaneously as a thermal reference medium.
Conventional micromechanical thermopile sensors with, for example, an infrared filter for gas detection are installed in TO05 or TO08 packages having optical windows. The thermopile element sits with its optically inactive side on the housing base, while the filters are adhesively bonded in the TO cap. Essential sensor properties such as sensitivity, time constant, and acquired spectral range can be influenced, during packaging of the sensors in hermetic metal housings of this kind, by selection of the filter medium and window material. This type of housing is very expensive and is also no longer hermetically sealed because of the necessary openings in the cap. There is generally no passivation of the bonding wires, which calls into question suitability for outdoor applications, in particular automobile applications, due to condensation, corrosion, etc.
Also known are completely injection-embedded or molded sensors, e.g. micromechanical acceleration sensors. Here the sensor element is secured (adhesively bonded or soldered) at its inactive rear side onto a carrier strip (lead frame). Wire-bond connections are used to create an electrical contact on contact surfaces (called “leads”) provided therefor on the rim of the chip mounting surface. The lead frame is then injection-embedded or molded using a plastic compound or molding compound. Because the molding compound is insufficiently transparent in the frequency range of interest for thermopile sensors for gas detection (e.g. in the infrared region at wavelengths greater than or equal to 4000 nm), optical access to the sensor would no longer exist with an installation technique of this kind (complete injection embedding).
Lastly, IC housings for power ICs, having an exposed chip installation region (called a “die pad”) on the rear side for purposes of better heat dissipation, are known.